Unless viewed in a pitch black environment, flat panel display systems deal with the phenomenon of ambient light reflection. The ambient light reflected from a display's surface can cause the display images to appear washed out, since the emitted image is competing with the reflected light bouncing off the display surface into the eyes of the observer. Attenuating this reflected light is important to gaining acceptable contrast ratio performance for flat panel displays. Contrast ratio is universally regarded as the strongest indicator of the visual quality of a display. It is highly desirable for a display to exhibit excellent contrast ratio even where ambient light is very high (such as in direct sunlight striking the display surface).
This goal, however, has become increasingly accessible. Many of the techniques used to improve contrast ratio by attenuation of ambient light reflection exhibit either limitations in their own performance, or undesirable side effects.
The advent of Broad Band Anti-Reflective (BBAR) coatings has served to reduce ambient light reflection to below 1% over a significant portion of the visible spectrum, but in the case of direct sunlight (where the energy to be attenuated is very high), the attenuation being achieved is still inadequate. Moreover, such BBAR coatings can cause a perceived tint in the image of the display, due to differential attenuation of the spectrum (strong attenuation of reds and greens, but weaker attenuation of reflected blues and violets).
The prior art contains various approaches to the problem of maximizing the contrast ratio of emissive and transmissive flat panel display systems deployed in environments where ambient light intensity is high (e.g., outdoors on a sunny day). In a darkened room, a given display may yield a measured contrast ratio of 1000:1 or greater, but outdoors it may measure only 3:1. Performance expectations have been lowered to the point where a 5:1 contrast ratio outdoors is considered good. Reduction of Fresnel reflections at crucial interfaces (the various layers comprising a display system) has led to the development of BBAR coatings as a first line of defense against excessive reflected light, which leads to washed out images and poor contrast ratio measurements.
Given the limitations usually applied (out of power efficiency considerations) to the emitted output of flat panel displays, the reduction in ambient reflections has traditionally taken center stage in the effort to improve contrast ratios. LCD-based displays, for example, have variously used absorbing layers, circular polarizer layers, and/or elaborate dark mask structures in front of the main display proper to further reduce the amount of ambient light reflected from the system. Some manufacturers, in desiring better performance than provided by emissive or transmissive systems, have switched to transflective architectures, presumably because it is easier to change architectures than to fight ambient light reflection. Manufacturers that stayed the course with their emissive and transmissive architectures faced major engineering tradeoffs. For example, increasing the image intensity of an LCD display involves high power consumption and reduced life spans for the backlight systems delivering that extra intensity to the display.